An already-controversial CD copy protection software hidden in Sony BMG music CDs and installed automatically on personal computers is now being exploited by virus writers, because of its ability to hide files on the computer.

Antivirus companies have so far been able to identify three separate trojan viruses written specifically to exploit the software. The names of the trojans are written to trigger the cloaking technology in Sony's software.

This means they can infect a PC completely undetected. A security analyst from Sophos said: "We wouldn't be surprised at all if we saw more malware that exploits what Sony has introduced." Sony offered a patch last week to decloak hidden files.